Fix return value comment on wfMemoryLimit()

It was changed incorrectly in I893ddd77.

Change-Id: I1f1e3c53a2cefe69b42ef338aa2a6b126e3d5526

includes/GlobalFunctions.php

@@ -3850,7 +3850,7 @@ function wfStripIllegalFilenameChars( $name ) {
 /**
  * Set PHP's memory limit to the larger of php.ini or $wgMemoryLimit
  *
- * @return int Prior memory limit
+ * @return int Resulting value of the memory limit.
  */
 function wfMemoryLimit() {
 	global $wgMemoryLimit;